Cells

A cell is the smallest unit that exhibits all of the
qualities associated with the living state. Cells must
obtain energy from an external source to carry on
such vital processes as growth, repair, and reproduction.
All of the chemical and physical reactions
that occur in a cell to support these functions constitute
its metabolism. Metabolic reactions are catalyzed
by enzymes. Enzymes are protein molecules
that accelerate biochemical reactions without being permanently altered
or consumed in the process. The structure of each enzyme (or any other
protein) is encoded by a segment of a deoxyribonucleic acid (DNA) molecule
referred to as a gene.

Molecular and cell biology are the sciences that study all life
processes within cells and at the molecular level. In doing so, these sciences draw upon knowledge from several scientific disciplines, including
biochemistry, cytology, genetics, microbiology, embryology, and
evolution.